1. Field of the Invention
The present invention relates generally to keyboard percussion instruments, such as marimbas, vibraphones and xylophones, which have resonators associated with tone bars.
Keyboard percussion instruments are particularly finicky in terms of their tuning and tone quality. They do not just go out of tune, they go out of tune in two different directions at once! In warm temperature and high humidity the tone bars go flat and the resonators go sharp. The opposite condition results in cool dry weather conditions. This adversely effects not only the pitch, as with any musical instrument, but also the tone quality of the instrument. This is because keyboard percussion instruments rely on sympathetic resonance of the resonator tube to the tone bar. If these two vibrating systems are not perfectly in tune, unmusical results are obtained.
Despite these problems with weather and tuning, keyboard percussion instruments are usually sold with non-movable force-fit metal stops in the resonator tubes. These permanent stops are prepositioned at the factory to resonate the above-suspended bar at a particular temperature and humidity level. To insure a perfect pneumatic seal and rigid structure, these oversized metal caps are inserted with a hydraulic press. Looser fitting caps are frequently glued or welded in position.
The position of these plugs is determined not only by the temperature and weather conditions at the point of manufacture, but also the taste of the designer and accidents and inconsistencies of manufacturing. When the instrument is played in an environment that exactly duplicates that for which it was tuned, (usually about 50% humidity and 72.degree. F.), these resonators perform admirably. However, a reduction of the ambient air temperature by as little as 4.degree. F. substantially reduces the volume potential of the instrument while increasing the apparent ring-time of the bar, adversely influencing the tone character of the combined bar/resonator system. Conversely, an increase of 4.degree. F. in the ambient temperature reduces the apparent ring-time of the bar/resonator system to a level that even a lay person can hear easily.
Until recently musicians have generally had to endure these shortcomings in performance. Even if the musician could take along all the wood-working or metal-working equipment to tune the tone bars at the performance site, this would not be a viable method to compensate for transitory weather conditions: tuning the tone bar requires removing material from the bar. No more than a few tunings can be performed before permanent loss of mass begins to be audible as loss of tone quality. Thus, the only way to bring these two sympathetically-vibrating systems into musical resonance is to change the effective length of the resonator tube.
In spite of great efforts expended by musical instrument manufacturers, finding an inexpensive and effective method by which the musician can quickly alter the pitch of resonator pipes, has been elusive. There is an inherent conflict in the function of a tunable stop; to produce the best musical tone, the stop needs to be air-tight, rigid and it should not camber away from a 90.degree. relationship to the wall of the resonator. Even when the seal is airtight, volume is greatly reduced and an unfocused tone is produced unless the seal is at the uppermost leading edge of the plug. To be useful for the musician, the plug must be quick to adjust over a one inch range and conveniently designed so that the player can hear the results while tuning. In summary, the problem musical instrument designers have encountered is that the easier the plug is to adjust, the less airtight and rigid the plug is. Thus, the sound is likely to be less musically satisfying. Last but most important, the design must be inexpensive enough to manufacture so that it they can be provided on all resonators of keyboard percussion instruments, not just the bottom few notes.
2. Description of the Prior Art
To those not skilled in the art of keyboard percussion instrument construction, a simple slide fit comes to mind as a possible solution to the above problems, but in actual practice, slight deformations of the roundness of resonators prevents a slide fit plug from producing a musically-useful result. For slightly different reasons traditionally-designed expansion plugs do not produce satisfactory results in musical instruments. These plugs usually have a top and bottom rigid plate-like structure with a soft, compressible intermediate layer between. Applying pressure on the two outer plates squeezes out the intermediate sealing layer. Not only does this seal the resonator at a point somewhat below the level of the leading edge, its inherent design allows the plug to camber when pressure is reduced. These two shortcomings produce an unfocused sound and reduce volume and richness of the tone character. For similar reasons O-ring plugs have not been successful: the seal is below the leading edge of the plug where the antinode of the vibrating column of air resides; cambering is common even when two O-rings are used per plug. Adjustability is reduced if the fit is tight enough to produce a rigid pneumatic seal.
In recent years the two largest manufacturers of keyboard percussion instruments in the world have introduced patented tunable resonators into their line of professional instruments. U.S. Pat. No. 4,570,525 assigned to Suzuki/Yamaha provides for `C` shaped expansion rings to provide radial expansion of a slotted cap against a tubular gasket. The `C` shaped expansion ring does not provide equal radial expansion for proper sealing against the entire circumference of the inside wall of the resonator. This reduces the maximum volume of this plug design. Further, the cost of manufacturing this system has limited its use to the bottom few notes of the most expensive instruments offered by this manufacturer.
In the present inventor's U.S. Pat. No. 4,941,386 a threaded cap assembly shortens and lengthens the effective length of the tube. This design has also proven to be very expensive to manufacture and as a result is only offered on the bottom twelve notes of the most expensive marimba offered by the assignee, The Selmer Company's Musser Division. Another shortcoming of this design is the speed of adjustment. The necessity of having thin-walled assembly introduced into the bottom of the tube so as to not adversely effect the tone of the resonator, limits the coarseness of the lead of the threads to about 20 per inch. This factor, along with the impingement of adjacent tubes and the anti-vibration fingers pressing on the cap, prevent the musician from turning the cap more than a 1/4 turn at a time. Thus, a one inch change of length on one resonator can require as many as 80 hand motions to accomplish. The present invention overcomes the above problems of cost, convenience and acoustics.